The invention relates generally to sensor systems, and more particularly to a sensor system for measuring a clearance between two objects through a ratiometric technique.
Various types of sensors have been used to measure the distance between two objects. In addition, these sensors have been used in various applications. For example, a turbine has a turbine blade that is disposed adjacent to a shroud. The clearance between the turbine blade and the shroud varies depending on the temperature of the turbine blade. For example, the clearance between the shroud and the turbine blade is greatest when the turbine is cold and gradually decreases as the turbine heats up. It is desirable that a gap or clearance between the turbine blade and the shroud be maintained for safe and effective operation of the turbine. A sensor may be disposed within the turbine to measure the distance between the turbine blade and the shroud. The distance may be used to direct movement of the shroud to maintain the desired displacement between the shroud and the turbine blade.
In certain applications, a capacitance probe is employed to measure the distance between two objects. The probe is located on one of the objects and measures a capacitance with respect to the other object for estimating the clearance between the two objects. Unfortunately, existing direct measurement techniques employing a single probe for a single target object are relatively inaccurate where the target object changes its geometry during the course of measurement. For example, in a rotating component, such as a turbine blade, the geometry and position of the blade may change depending on various conditions. Such changes may result in a drift in the calibration of the probe. Further, noise due to thermal distortions in the wire connecting the probe to a clearance control unit may also results in inaccurate measurements.
Moreover, in certain applications such as gas turbines, such sensor systems are typically employed to measure clearances between parts during design and offline testing. Unfortunately, these sensor systems are not workable for in-service measurements due to the noise and drift generated by changes in geometry of the parts. Instead, in-service clearance control is based on the clearance measurements previously taken during design and testing of the parts. As the parts become worn during service, the offline measurements become ineffective for in-service clearance control.
Accordingly, a need exists for providing a sensor system that provides an accurate measurement of clearance between two objects by minimizing the effect of calibration drift and noise in a system. It would also be advantageous to provide a self-calibrating sensor system that could be employed for accurate clearance measurement for parts in operation.